This invention relates to electronically commuted switched reluctance machines and more particularly to continuous rotation motors operated by sources of polyphase electric energy.
Switched reluctance motors are well known in the art. These motors have a stationary member, typically called a stator, and a movable member, typically called a rotor. The rotor and stator are oriented such that they move relative to each other. A typical stator includes a yoke supporting a plurality of magnetically permeable poles circumferentially spaced and having gaps therebetween. A typical rotor includes a magnetically permeable body comprised of laminations of magnetically permeable steel forming two or more poles circumferentially spaced and having gaps therebetween. The rotor is disposed relative to the stator such that their respective poles pass closely adjacent, when the rotor is moved relative to the stator, i.e., the poles of the rotor move in spaced relation to the poles of the stator. The motor has phase windings on the poles of the stator but not on the poles of the rotor. Switched reluctance motors rely on polyphase electronic commutation to excite these phase windings in proper sequence to cause movement of the rotor relative to the stator. Specifically, excitation of the phase windings produces on the stator a pole pair having a north pole and a south pole. These phase windings create a magnetic flux path that passes through the polarized pole pairs, the rotor and the yoke of the stator, i.e., a magnetic circuit. In response to flux passing therethrough, the rotor moves to bring a pair of rotor poles into a minimum reluctance position relative to the polarized pair of stator poles. This minimum reluctance position corresponds to the maximum inductance of the energized phase winding. A feature common to two phase SR motors is that the rotor is typically configured to optimize rotation in one direction. Advantages of switched reluctance motors (hereinafter "SR" motors) are that they are efficient in converting electrical energy into mechanical work, they are reliable because of their mechanical simplicity and they are capable of significant rotational speeds, i.e., 100,000 RPM. Additionally, SR motors are inexpensive to produce, they are rugged and robust and do not require brushes or slip rings.
A number of common SR motor configurations and electronic commutation combinations exist to fulfill certain end use requirements. Some polyphase source and stator/rotor combinations include, without limitation, two phase 8/4 motor; three phase 6/4 motor; four phase 8/6 motor and a five phase 10/8 motor. One reason for increasing the number of stator and rotor poles and for having higher numbers of phases is to increase the number of electronic phase commutations per revolution thereby minimizing torque dips or torque ripple between the phases.
Torque in an SR motor is related to changing inductance (dL) of energized phase windings as a function of rotor position. Inductance in an SR motor decreases or increases as the poles of the rotor move into or out-of alignment with the poles associated with the energized stator windings, i.e., as the rotor-stator system moves into or out-of a minimum reluctance position. Stated differently, torque is produced when there is a change in inductance as a function of angular position, i.e., dL/d.theta.; positive torque being produced when the inductance of an energized phase increases and negative torque being produced when the inductance of an energized phase decreases.
A problem with prior art two phase SR motors is that at certain angular positions of the rotor relative to the stator, the torque experienced by the rotor is zero or a very small percentage of maximum torque. This position of little or no torque results from the poles of the rotor and the stator being positioned relative to each other such that insufficient flux from an energized stator pole pair passes through a pair of rotor poles to cause relative motion therebetween. Attempts at overcoming this problem included modifying the geometries of the rotor poles such that portions of the rotor pole are in sufficient flux communication with an energized stator pole to impart torque to the rotor.
One such geometry includes a stepped gap rotor wherein a first portion of the face of a rotor pole coming into flux communication with the energizing stator pole forms a gap with the face of the stator pole having a first gap space. The second portion of the face of the rotor pole coming into flux communication with the face of the stator pole forms a second gap that is narrower than the first gap space; the transition between the first gap space and the second gap space being a step.
Another geometry includes a snail-cam design wherein the face of the rotor pole tapers such that the gap between the rotor and the stator becomes progressively smaller as the rotor rotates into minimum reluctance position with respect o the stator. For these pole geometries the faces of the rotor poles are widened such that the first portion of the rotor pole extends towards an adjacent deenergized stator pole when the second portion of the rotor pole is in a minimum reluctance position with an energized stator pole. These various rotor pole geometries eliminate positions of zero torque in a two phase motor, however, such rotor geometries are unable to produce consistent torque throughout the rotation of the rotor. This inconsistent torque, or torque ripple, produced by prior art two phase SR motors is unacceptable for certain applications, such as washing machines, fluid pumps, traction motors, position servos and the like, wherein significant torque may be required at any position of the rotor relative to the stator.
An attempt at overcoming torque ripple in SR motors includes increasing the number of commutation phases to 3 or more. It is well known that torque ripple generally decreases with an increasing number of motor phases. Specifically, 3 phase motors generally have less torque ripple than 2 phase motors, 4 phase motors have less torque ripple than 3 phase motors and so on. The decrease in torque ripple with increasing phases results from the dL/d.theta. from one phase being non-zero before the dL/d.theta. from an immediately preceding phase becoming zero. Thus, increasing the number of phases to 3 or more produces closely adjoining or overlapping dL/d.theta. such that the rotor experiences torque from the energization of one phase before the termination of torque from the energization of another phase. This continuity of torque or overlap in torque between phases of an SR motor results in a more continuous torque having less torque ripple. Problems with SR motors having 3 or more phases, however, are the increased quantity of components for the commutation electronics, and consequently the cost thereof; the increased number of connections between the commutation electronics and the phase windings; the increased resolution of position sensors required to resolve the position of the rotor for the electronic commutation; and more acoustic noise over 2 phase SR motors.
It is the object of the present invention to provide a new and improved SR motor that overcomes the above-referenced problems and others.